Method of producing formaldehyde directly from methane

ABSTRACT

The present invention provides a silica-supported 12-molybdosilicic acid catalyst comprising a silica carrier and at least 10% by weight, based on the silica amount, of 12-molybdosilicic acid supported on the carrier and a method of producing formaldehyde directly from a mixed gas of methane and oxygen in the presence of the particular catalyst. Formaldehyde can be produced at a high yield in the present invention without requiring a reforming process of methane with water vapor that consumes a large amount of energy. In addition, attentions are also paid to the air pollution and water contamination problems in the present invention.

BACKGROUND OF THE INVENTION

The present invention relates to a method of producing formaldehyde,particularly to a novel catalyst that permits producing formaldehydedirectly from methane at a high yield and a method of producingformaldehyde by using the novel catalyst.

Formaldehyde is produced by partial oxidation reaction of methanol. Halfthe methanol produced in an amount of one million tons in a year is usedas a raw material for the production of formaldehyde. The producedformaldehyde is used as a raw material of synthetic resins such asphenolic resins and urea resins or as a raw material of variousmedicines.

Methanol is synthesized from hydrogen and carbon monoxide obtained bywater vapor reforming reaction of methane. The conventional process ofproducing formaldehyde is as given below:

Methane→H₂/CO→Methanol→Formaldehyde

The reaction for preparing H₂/CO from methane is an endothermic reactionusing a large amount of high temperature water vapor, which is one oftypical processes consuming a large amount of energy. On the other hand,the reaction for synthesizing methanol from H₂/CO is an exothermicreaction. In order to prevent the reaction heat from being generatedexcessively, the CO conversion rate must be suppressed to about 10% inoperating the process unit. Also, the conversion rate of methanol mustbe suppressed in operating the process unit in the production offormaldehyde by partial oxidation of methanol in order to suppressformation of carbon dioxide and carbon monoxide. In short, theconventional process of producing formaldehyde is a process consuming alarge amount of energy and requiring a very complex operation of theprocess unit.

In order to avoid the large energy-consuming process in the productionof formaldehyde, it is necessary to develop a new producing process thatpermits producing formaldehyde without involving the step of water vaporreformation of methane to produce H₂/CO.

It is considered theoretically possible to produce methanol andformaldehyde by partial oxidation of methane, i.e., direct synthesisfrom methane, as suggested by chemical reaction formulas given below:

CH₄+1/2O₂→CH₃OH, CH₄+O₂→HCHO+H₂O

Therefore, vigorous researches are being made over more than these 50years on the method of directly synthesizing methanol or formaldehydefrom methane in research institutes over the world. Since a catalyst isrequired for the reactions given above, the major portion of theseresearches has been concentrated on the development of an effectivecatalyst. For example, catalysts having molybdenum oxide, vanadiumoxide, chromium oxide, etc. supported by silica are reported in, forexample, “Chemistry Letter, 1997, p31-32” and “Catalyst Today, 45,p29-33(1998)”.

However, the yield of methanol or formaldehyde is very low, i.e., lessthan 1% in general, even in the presence of these catalysts. It is saidamong the researchers in this field that it is difficult for the yieldof methanol or formaldehyde to exceed 4%. In other words, the yield of4% is said to be a wall that cannot be broken in the direct synthesis ofmethanol or formaldehyde from methane. Further, the methane conversionrate of at least 10% is required for putting the process to practicaluse. In conclusion, a process for direct conversion from methane intoformaldehyde with a practical yield of formaldehyde has not yet beendeveloped.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to produce formaldehyde with ahigh yield directly from methane by a process that does not involve awater vapor reforming step of methane, which is a step consuming a largeamount of energy, and that does not bring about an air pollution orwater contamination problem. To achieve the object, the presentinvention provides a novel catalyst and a method of producingformaldehyde by using the novel catalyst.

The present inventors have conducted an extensive research on a catalystexcellent in its activity of partially oxidizing methane and on theconditions of the reaction carried out in the presence of the particularcatalyst, and found that a silica-supported 12-molybdosilicic acidcatalyst, in which 12-molybdosilicic acid is supported on silica, isexcellent in its activity of partially oxidizing methane and, thus, isvery effective when used as a catalyst in the synthesis of formaldehydedirectly from methane. The formaldehyde yield is markedly improved ifthe reaction for direct synthesis of formaldehyde from methane iscarried out under a water vapor atmosphere in the presence of theparticular catalyst.

According to the present invention, there is provided a silica-supported12-molybdosilicic acid catalyst, comprising a silica carrier and atleast 10% by weight, based on the silica amount, of 12-molybdosilicicacid supported on the carrier.

It is desirable for the silica carrier to have a specific surface areaof at least 500 m²/g.

The present invention also provides a method of directly producingformaldehyde from a mixed gas of methane and oxygen in the presence of acatalyst, wherein the catalyst comprises a silica carrier and at least10% by weight, based on the silica amount, of 12-molybdosilicic acidsupported on the carrier.

It is desirable for the volume ratio of methane to oxygen of the mixedgas to fall within a range of between 9/1 and 4/6 and to carry out thereaction in the presence of the catalyst at 550 to 650° C.

It is also desirable to supply water vapor into the mixed gas of methaneand oxygen in an amount of 40 to 80% by volume of the total volume ofthe mixed gas and water vapor.

The present invention makes it possible to synthesize formaldehydedirectly from methane at a high yield. This suggests that theconventional process consuming a large amount of energy need not beemployed for the production of formaldehyde, leading to a prominentenergy saving in the chemical industries.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 schematically shows an apparatus for evaluating the activity of asilica-supported 12-molybdosilicic acid (SMA) catalyst;

FIG. 2 is a graph showing the relationship between the catalyticactivity and the water vapor supply amount;

FIG. 3 shows an infrared ray spectrum denoting the thermal decompositionbehavior of a 27 wt % SMA/SiO₂ catalyst;

FIG. 4 is a graph showing the relationship between the catalyticactivity and the amount of SMA supported on the carrier;

FIG. 5 is a graph showing the relationship between the catalyticactivity and methane/oxygen volume ratio;

FIG. 6 is a graph showing the relationship between the catalyticactivity and the specific surface area of silica used as a carrier;

FIG. 7 is a graph showing the relationship between the catalyticactivity and the reaction temperature;

FIG. 8 is a graph showing the durability of the catalyst with 9/1 ofmethane/oxygen ratio; and

FIG. 9 is a graph showing the durability of the catalyst with 6/4 ofmethane/oxygen ratio.

DETAILED DESCRIPTION OF THE INVENTION

In the present invention, reaction between methane and oxygen is carriedout in the presence of a catalyst for producing formaldehyde directlyfrom methane. The catalyst used in the present invention is prepared byhaving 12-molybdosilicic acid supported on a silica carrier.

The silica-supported 12-molybdosilicic acid catalyst used in the presentinvention can be prepared by an impregnation method as follows.Specifically, 12-molybdosilicic acid (hereinafter referred to as “SMA”)is sufficiently dissolved in pure water at room temperature. Then, asilica powder is dipped in the solution, followed by evaporating thewater such that the catalyst is not dried completely. If the catalyst iscompletely dried and heated at 350°C. or more, SMA within the catalystis thermally decomposed into silica and molybdenum oxide, as shownbelow:

H₄SiMO₁₂O₄₀→SiO₂+12MoO₃+2H₂O

Therefore, the water should be evaporated on, preferably, a water bathwhile heating and stirring the solution such that the catalyst is notdried completely.

After the water evaporation, the catalyst is further dried for obtainingthe catalyst of the present invention.

SMA used as a raw material is a compound having a molecular formula ofH₄SiMo₁₂O₄₀. In the present invention, it is possible to use SMAavailable on the market. The silica powder used as a carrier shoulddesirably have a high purity and a specific surface area of at least 500m²/g. If the specific surface area is smaller than 500 m²/g, SMA isagglomerated on the surface of the silica carrier so as to inhibit thepartial oxidation reaction of methane. The silica powder can be preparedby a known method. For example, a silica gel obtained by hydrolyzingethyl silicate is dried and calcined to obtain a desired silica powder.A silica carrier having a desired specific surface area can be obtainedby controlling the pH value in the hydrolyzing step. It should be notedthat the pores present in the silica obtained by hydrolyzing ethylsilicate have an average diameter of about 40 Å. On the other hand, SMAmolecules have an average diameter of about 28 Å even under a watervapor atmosphere described herein later. It follows that the SMAmolecules can be held within the pores of the silica carrier, making itpossible to suppress elution of the water-soluble SMA molecules so as toimprove the durability of the catalyst. However, a silica powderavailable on the market can also be used satisfactorily in the presentinvention as the silica carrier.

It is desirable for SMA to be supported on the silica carrier in anamount of at least 10% by weight, preferably 10 to 50% by weight, andmost preferably 25 to 40% by weight, based on the weight of the silicacarrier. If the amount of SMA supported on the carrier is less than 10%by weight, the methane conversion rate is low, resulting in failure toobtain formaldehyde at a sufficiently high yield. If the supported SMAamount exceeds 50% by weight, however, it is impossible to obtainformaldehyde at a high yield conforming with the supported SMA amount.It should be noted in this connection that a regeneration reaction ofSMA given below, which will be described hereinlater, takes place undera water vapor atmosphere:

SiO₂+12MoO₃+2H₂O→H₄SiMo₁₂O₄₀

What should be noted is that the SMA formation amount by theregeneration reaction given above is limited to a certain value. As aresult, formaldehyde cannot be obtained at a high yield conforming withthe SMA amount where the amount of SMA supported on the carrier exceeds50% by weight, as pointed out above.

Incidentally, the amount of SMA supported on the carrier can becontrolled as desired by controlling the SMA concentration in theaqueous solution of SMA in the catalyst preparation method describedabove.

For producing formaldehyde in the present invention, a mixed gas ofmethane and oxygen is brought into contact with the catalyst of thepresent invention.

For example, a mixed gas of methane and oxygen is passed through a bedof the catalyst of the present invention so as to carry out reactionbetween methane and oxygen.

In the present invention, it is desirable to supply water vapor into themixed gas of methane and oxygen so as to carry out the formaldehydesynthesizing reaction under a water vapor atmosphere. It should be notedin this connection that SMA is poor in heat resistance. If heated to350° C. or more, SMA is thermally decomposed easily to form silica andmolybdenum oxide. If heated to the formaldehyde synthesizingtemperature, e.g., 600° C., SMA is thermally decomposed substantiallycompletely into silica and molybdenum oxide, though the heat resistanceof SMA is certainly improved if SMA is supported on silica. However, thethermal decomposition and regeneration of SMA proceed in equilibriumunder a water vapor atmosphere, with the result that SMA is constantlypresent during the reaction for synthesizing formaldehyde. In otherwords, the thermal decomposition of SMA during the reaction issuppressed in the presence of water vapor so as to permit SMA to producea catalytic function inherent in SMA.

The reaction for synthesizing formaldehyde proceeds satisfactorily ifthe volume ratio of methane/oxygen in the mixed gas falls within a rangeof between 9/1 and 4/6. It is more desirable for the volume ratio ofmethane/oxygen to fall within a range of between 7/3 and 6/4 in order toobtain formaldehyde efficiently in view of the methane conversion rateand the formaldehyde selectivity.

In the present invention, the reaction for synthesizing formaldehydeshould desirably be carried out at 550 to 650° C. If the reactiontemperature is lower than 550° C., the methane conversion rate islowered, leading to a low formaldehyde yield. Also, if the reactiontemperature is higher than 650° C., the selectivity of carbon dioxide orcarbon monoxide is increased so as to lower the formaldehyde yield. Morepreferably, the reaction temperature should be 580 to 620° C. Thereaction mechanism given below is considered to account for the reactiontemperature specified in the present invention:

CH₄(g)+H⁺(ad)→CH₅ ⁺(ad)  (1)

O₂(g)+2s→20⁻(ad)  (2)

CH₅ ⁺(ad)+20⁻(ad)→CH₃O⁺+H₂O(g)+2s  (3)

H₂O(g)+2s→OH⁻(ad)+H⁺(ad)  (4)

CH₃O⁺(ad)+OH⁻(ad)→HCHO(g)+H₂O(g)+2s  (5)

CH₃O(ad)+2.5O⁻(ad)→CO₂(g)+1.5H₂O(g)+3.5s  (6)

CH₃O(ad)+1.5O⁻(ad)→CO(g)+H₂O(g)+2.5s  (7)

In the reaction formulas given above, the mark “(ad)” represents anadsorbed state, and “s” denotes the oxygen active point on the SMAcatalyst. Formaldehyde is formed by reactions (1) to (5). However,reactions (6) and (7) also take place to form carbon dioxide and carbonmonoxide. Also, the mark “H⁺ (ad)” represents a proton present on SMA. Asingle SMA molecule has four protons. For synthesizing formaldehyde bypartial oxidation of methane, it is absolutely necessary to form anadsorbed methoxy group “CH₃O(ad)”. In the case of using an SMA catalyst,it is a strong likelihood that a carbonium cation “CH₅ ⁺ (ad)” is formedfirst by the action of H⁺ (ad) and, then, the carbonium cation CH₅ ⁺(ad) is converted into the adsorbed methoxy group CH₃O(ad). Where thereaction temperature is low, the activation and adsorption of methaneshown in reaction (1) does not proceed. On the other hand, where thereaction temperature is high, the side reactions (6) and (7) arepromoted so as to inhibit formation of methanol or formaldehyde.

In the present invention, water vapor should be supplied in an amount of40 to 80% by volume of the total volume of the mixed gas of methane andoxygen and the water vapor. Preferably, water vapor should be suppliedin an amount of 60 to 70% by volume of the total volume in order toobtain formaldehyde efficiently in view of the methane conversion rateand the formaldehyde selectivity.

EXAMPLES

A novel catalyst excellent in partial oxidation activity of methane,which is prepared by having SMA supported on silica, and a method ofproducing formaldehyde in the presence of the particular catalyst willnow be described with reference to Examples.

First of all, a method of preparing a silica powder used as a carrier ofthe catalyst will now be described as Preparation Example 1.

Preparation Example 1 Preparation of Silica Powder Used as a Carrierpre

A silica gel obtained by hydrolyzing ethyl silicate was dried at 110° C.for 10 hours, followed by calcining the dried silica gel at 600° C. for3 hours so as to prepare a high purity silica used as a carrier forpreparing a silica-supported SMA catalyst of the present invention.Three kinds of silica carriers differing from each other in the specificsurface area as shown in Table 1 below were prepared by changing the pHvalue in the hydrolyzing step.

TABLE 1 Silica powders having different specific surface areas pH inSpecific hydrolyzing surface step area (m²/g) Silica powder 1 1.0 390Silica powder 2 2.0 570 Silica powder 3 2.5 740

Preparation of a silica-supported SMA catalyst will now be described asExample 1.

Example 1 Preparation of Silica-Supported SMA Catalyst

A silica-supported SMA catalyst was prepared by an impregnation methodas follows.

In the first step, 5 g of an SMA powder available on the market wasdissolved sufficiently in 50 milliliters (mL) of pure water at roomtemperature, followed by transferring the resultant SMA solution into anevaporating dish and subsequently dipping 20 g of the silica powderprepared in Preparation Example 1 in the SMA solution. Then, thesilica-containing solution was heated at about 50° C. above a water bathwhile stirring the solution to evaporate water such that the catalystwas not dried completely. The catalyst having water evaporated therefromwas put in a dryer kept at about 110° C. for further drying for 10hours. The resultant catalyst, which was found to contain 20% by weightof SMA supported on the silica carrier, was put in a hermetically sealedpolyethylene bag and stored in a desiccator.

A method of evaluating the activity of the silica-supported SMA catalystprepared in Example 1 and a reaction apparatus for evaluating theactivity by the particular method will now be described as Example 2.

Example 2 Method of Evaluating Activity of Silica-Supported SMA Catalystand Reaction Apparatus for Activity Evaluation

FIG. 1 schematically shows a reaction apparatus for evaluating theactivity of the silica-supported SMA catalyst of the present invention.As shown in the drawing, a methane bomb 1, an oxygen bomb 2 and a watersupply device 4 are connected to a gas mixer 3 through pipes. Also, thegas mixer 3 is connected to a reaction tube 11 through a pipe 20. Thepipe 20 is branched upstream of the reaction tube 11, and a branchedpipe 21 connected at one end to the branched portion of the pipe 20communicates at the other end with a valve 8. A pipe 22 communicates atone end with the outlet port of the reaction tube 11 and is connected atthe other end to gas chromatographs 9 and 10 through branched pipes 22a, 22 b, respectively. The branched pipe 21 communicates with the pipe22 via the valve 8. A catalyst 6 is loaded in the reaction tube 11, anda heater 7 is arranged to surround the reaction tube 11. Further, athermocouple 5 is inserted into the reaction tube 11.

Methane and oxygen are supplied from the bombs 1 and 2 into the gasmixer 3 at supply rates of, generally, 1.8L (liters)/h and 0.2 L/h,respectively.

The water supplied from the water supply device 4 is converted intowater vapor within the gas mixer 3 heated to 250° C. The water vapor ismixed completely before entering the catalyst layer with the methane andoxygen by the action of ceramic pieces (not shown) such as glass beads(5 mmφ) loaded in the mixer 3 . In order to prevent occurrence of apulsating flow of water during the reaction, a press fitting device fora liquid chromatography is used as the water supply device 4. The watervapor supply rate can be controlled freely within a range of between 0.2L/h and 4 L/h.

The mixed gas is supplied into the gas chromatographs 9 and 10 throughthe branched pipes 22 a and 22 b, respectively. Carbosieve S-II isloaded in the column of the gas chromatograph 9 for measuring mainlymethane, carbon monoxide and carbon dioxide. On the other hand, APS-201is loaded in the column of the gas chromatograph 10 for measuring mainlymethanol, formaldehyde and water. If the valve 8 mounted to the branchedpipe 21 is closed, the mixed gas is introduced into the reaction tube 11having a diameter of 10 mm and loaded with 1.5 g of the catalyst 6. Thecatalyst layer 6 is heated by the heater 7 set at 550 to 650° C., andthe temperature within the reaction tube 11 is measured by thethermocouple 5. The formed gas passing through the catalyst layer 6 isalso introduced into the gas chromatographs 9 and 10 for the compositionanalysis like the mixed gas flowing through the branched pipe 21. Themain component of the formed gas is formaldehyde. However, about 5% byweight of methanol based on the amount of formaldehyde is also containedin the formed gas. Therefore, formaldehyde and methanol are denotedtogether by the expression “methanol/formaldehyde” in the followingdescription.

The activity of the catalyst was evaluated by calculating the methaneconversion rate and the selectivity of each of methanol/formaldehyde,carbon dioxide and carbon monoxide within the formed gas by the formulasgiven below:

Methane Conversion Rate=P/the number of mols of introduced methane;

Selectivity of Formed Compound X=the number of mols of formed compoundX/P

where P denotes the total number of mols of(methanol/formaldehyde+CO+CO₂), and X denotes the number of mols ofmethanol/formaldehyde, CO or CO₂.

The influences given by the water vapor addition amount to the activityof the silica-supported SMA catalyst will now be described as Example 3.

Example 3 Dependence of Catalyst Activity on Water vapor Addition Amount

A catalyst having 27% by weight of SMA supported on the silica powder 2prepared in Preparation the catalyst was prepared as in Example 1. Thecatalyst thus prepared is abbreviated herein as “27 wt % SMA/SiO₂”. Thecatalytic activity was evaluated by the method of Example 2.Specifically, the methane conversion rate and the selectivity of each ofmethanol/formaldehyde, CO and CO₂ were calculated under the conditionsgiven below:

Catalyst loading amount: 1.5 g

Methane flow rate: 1.8 L/h

Oxygen flow rate: 0.2 L/h

Reaction temperature: 600° C.

Water vapor addition rate: 0.5 L/h to 3.5 L/h

The yield of methanol/formaldehyde was also calculated based on thecalculated values of the methane conversion rate and the selectivity.The results are shown in Table 2 and in FIG. 2.

TABLE 2 Change in catalytic activity caused by change in water vaporaddition amount Water vapor addition rate (L/h) 0.5 1.0 1.5 2.0 2.5 3.03.5 Water vapor 20 33 43 50 56 60 64 partial pressure in total gas (%)Methane conversion 3.0 3.0 3.1 4.0 8.4 10.5 11.3 rate (%) Methanol/ 37.159.0 45.3 67.2 77.5 81.0 83.9 formaldehyde selectivity (%) COselectivity 24.2 20.3 22.5 12.4 4.7 9.1 7.8 (%) CO₂ selectivity 38.720.7 32.2 20.3 17.8 9.9 9.0 (%) Methanol/ 1.11 1.75 1.42 2.70 6.53 8.529.44 formaldehyde yield (%) *27 wt % SMA/SiO₂, specific surface area ofsilica: 570 m²/g *Methane flow rate: 1.8 L/h, oxygen flow rate: 0.2 L/h*Reaction temperature: 600° C.

As apparent from the experimental data, the methane conversion rate, themethanol/formaldehyde selectivity and the methanol/formaldehyde yieldare increased with increase in the water vapor amount within the totalgas. Particularly, it has been confirmed that, if the water vaporpartial pressure is increased to 60% or more, the methane conversionrate is rapidly increased and the methanol/formaldehyde selectivity isincreased to exceed 80%, supporting that methane is efficientlyconverted into methanol/formaldehyde. It has also been confirmed thatthe methanol/formaldehyde yield exceeds 8%, which is markedly higherthan 4% that was regarded as an unbreakable wall in this technicalfield. The amount of methanol in the methanol/formaldehyde was found tobe not larger than 5% by weight of the amount of formaldehyde under eachof the water vapor partial pressures tested. This was also the case withany of the following Examples.

As described above, it has been found that the water vapor addition tothe methane/oxygen mixed gas is highly effective for improving theformaldehyde field, and that the effect produced by the water vaporaddition is prominently increased if the water vapor partial pressure is60% or more. In order to clarify the role played by the water vapor forimproving the catalytic function, the structure of the SMA catalyst inthe presence of water vapor was studied by an infrared spectroscopicanalysis in Example 4 that follows.

Example 4 Role Played by Water vapor for Improving Catalytic Function

The catalyst used in Example 3, i.e., 27 wt % SMA/SiO2, was diluted witha KBr powder to form a pellet sample for the IR spectroscopic analysis.The pellet sample thus formed was put in a heating type infraredspectroscopic cell for measuring the infrared absorption spectrum. FIG.3a shows the changes in the infrared absorption spectra when the pelletsample was heated under the air atmosphere. Two absorption peaksobserved at 907 cm⁻¹ and 954 cm⁻¹ are inherent in SMA. If the pelletsample is heated to 400° C. or more, these two peak intensities areattenuated and a single absorption peak is newly observed at 1000 cm⁻¹.The new absorption peak is characteristic of molybdenum oxide (MoO₃). Ifthe sample is heated to 600° C., the absorption peaks ascribed to SMAdisappear substantially completely, and only the absorption peakascribed to MoO₃ can be observed.

In general, SMA is poor in heat resistance. If heated to 350° C. orhigher, SMA is easily decomposed to form silica and molybdenum oxide asgiven below:

H₄SiMO₁₂O₄₀→SiO₂+12MoO₃+2H₂O

On the other hand, the experimental data given in FIG. 3a suggest thatthe heat resistance of SMA may be improved if SMA is supported onsilica. Even in this case, SMA is thermally decomposed substantiallycompletely into silica and molybdenum oxide if heated at 600° C.

The sample thermally decomposed completely was cooled to roomtemperature and left to stand under a water vapor atmosphere for 12hours, followed by observing the absorption spectra. FIG. 12b shows theresults. In this case, two absorption peaks were observed at 907 cm⁻¹and 954 cm⁻¹, and an absorption peak was not observed at 1000 cm⁻¹. Thissuggests that SMA was gradually regenerated by the treatment with watervapor. Incidentally, it is known to the art that a small amount of SMAis formed if a silica powder and a molybdenum oxide powder are mixed andstirred within water, as described in, for example, “J. M. Tatibouet, etal., J. Chem. Soc., Chem. Commun., 1260(1988)” and “C. R. Deltcheff, etal., J. Catal., 125, 292 (1990)”:

SiO₂+12MoO₃+2H₂O→H₄SiMO₁₂O₄₀

If heated, the regenerated SMA begins to be thermally decomposed at 400°C. and is thermally decomposed completely at 600° C. to form silica andmolybdenum oxide, as in FIG. 3a.

What should be noted is that, under the water vapor atmosphere, thethermal decomposition and regeneration of SMA proceed in equilibrium,with the result that SMA is constantly present during the reaction. Inother words, the presence of water vapor serves to suppressdecomposition of SMA during the reaction. Under the circumstances, watervapor is considered to regenerate SMA during the reaction so as topermit SMA to produce a catalytic function inherent in SMA.

The influences given by the supported amount of SMA to the catalyticactivity will now be described as Example 5.

Example 5 Dependence of Catalytic Activity on Supported Amount of SMA

It has been confirmed in Example 3 that the yield ofmethanol/formaldehyde is markedly increased where the water vapor supplyrate is set at 3.0 to 3.5 L/h (water vapor partial pressure of 60 to64%). In Example 5, an experiment was conducted under a water vaporsupply rate of 3.5 L/h by changing the amount of SMA supported on silicapowder 2 in order to observe the change in the catalytic activity causedby the change in the amount of the supported SMA. The other reactingconditions were equal to those in Example 3, as given below:

Methane flow rate: 1.8 L/h

Oxygen flow rate: 0.2 L/h

Reaction temperature: 600° C.

The amount of the supported SMA was changed within a range of 6 to 36%by weight. The experimental data are shown in Table 3 and FIG. 4.

TABLE 3 Change in catalytic activity caused by change in amount ofsupported SMA Amount of supported SMA (% by weight) 6 12 18 27 36Methane 4.3 6.5 7.9 11.3 11.0 conversion rate (%) Methanol/ 84.8 71.483.2 83.9 86.6 formaldehyde selectivity (%) CO 2.6 12.9 3.6 7.8 4.8selectivity (%) CO₂ 12.6 15.7 13.2 9.0 8.5 selectivity (%) Methanol/3.67 4.66 6.60 9.44 9.56 formaldehyde yield (%) *Specific surface areaof silica: 570 m²/g *Methane/oxygen/water vapor flow rate (L/h):1.8/0.2/3.5 *Reaction temperature: 600° C.

The experimental data indicate that the methane conversion rate isincreased with increase in the supported amount of SMA, leading to anincreased yield of methanol/formaldehyde, though the selectivity ofmethanol/formaldehyde is not appreciably changed by the change in thesupported amount of SMA. Particularly, a prominently high yield ofmethanol/formaldehyde, i.e., about 10%, was obtained where the supportedamount of SMA exceeded 18% by weight.

The influences given by the methane/oxygen ratio of the methane/oxygenmixed gas to the catalytic activity will now be described as Example 6.

Example 6 Dependence of Catalytic Activity on Methane/Oxygen Ratio

The direct synthesis of formaldehyde by partial oxidation of methaneproceeds as follows:

CH₄+O₂→HCHO+H₂O

Clearly, it is stoichiometrically desirable for the methane/oxygen ratioof the methane/oxygen mixed gas used in the reaction to be 1/1. In eachof the experiments conducted in the Examples described above, themethane/oxygen ratio was set at 9/1. In Example 6, 1.5 g of 27 wt %SMA/SiO₂ catalyst was used, and the reaction was carried out under awater vapor flow rate of 3.5 L/hr and the reaction temperature of 600°C., while changing the methane/oxygen ratio within a range of 9/1 to 4/6for evaluating the catalytic activity. On the other hand, the flow rateof the methane/oxygen mixed gas was set constant at 2.0 L/h. Theexperimental data are shown in Table 4 and FIG. 5.

TABLE 4 Change in catalytic activity caused by change in methane/oxygenratio Methane/oxygen volume ratio 9/1 7/3 6/4 4/6 Methane 11.3 13.9 19.712.2 conversion rate (%) Methanol/ 83.9 88.1 87.6 84.1 formaldehydeselectivity (%) CO 7.8 5.0 6.2 5.1 selectivity (%) CO₂ 9.0 7.0 6.2 10.8selectivity (%) Methanol/ 9.44 12.25 17.25 10.22 formaldehyde yield (%)*Sum of methane/oxygen: 2.0 L/h *Water vapor flow rate: 3.5 L/h (watervapor partial pressure: 64%) *Amount of supported SMA (wt %): 27 wt %*Reaction temperature: 600° C.

As shown in Table 4, the methane/oxygen volume ratio was changedstepwise from 9/1 to 4/6. In other words, the oxygen content of themethane/oxygen mixture was increased stepwise. The methanol/formaldehydeyield was found to increase with increase in the oxygen content of themethane/oxygen mixture to reach 17.25% when the methane/oxygen volumeratio was set at 6/4. Where the methane/oxygen volume ratio was heldwithin a range of between 9/1 and 6/4, the methanol/formaldehydeselectivity of the reaction mixture was held substantially constant at85% or more. Also, the selectivity of any of carbon monoxide and carbondioxide was lower than 10%. Where the methane/oxygen volume ratio wasset at 4/6, the methane conversion rate was decreased and theselectivity of the carbon dioxide was increased. As a result, the yieldof methanol/formaldehyde was also lowered, though themethanol/formaldehyde yield was very high even in this case, i.e.,10.22%.

The effect given to the catalytic activity by the specific surface areaof the silica carrier used in the production of the silica-supported SMAcatalyst will now be described as Example 7.

Example 7 Dependence of Catalytic Activity on Specific Surface Area ofSilica Carrier

A single SMA molecule has a diameter of about 28 Å and, thus, has across sectional area of about 600 Å². Also, SMA has a molecular weightof about 1824. Therefore, in the 27 wt % SMA/SiO₂ catalyst, 0.27 g ofSMA, i.e., 0.9×10²⁰ SMA molecules, are supported by 1 g of silica. Itshould be noted that the sum of the cross sectional areas of 0.9×10²⁰SMA molecules is 5.4×10²²Å², i.e., 540 m². On the other hand, silicapowder 2 shown in Table 1 was used as a silica carrier. As shown inTable 1, silica powder 2 has a specific surface area of 570 m²/g. Itfollows that, in the 27 wt % SMA/SiO₂ catalyst, the entire surface ofthe silica carrier is covered substantially completely with a monolayerof the SMA molecules. In other words, if a larger amount of SMAmolecules are supported on the silica carrier, the SMA molecules arecaused to form a plurality of layers on the silica surface. According tothe experimental data obtained in Example 5, the methanol/formaldehydeyield remains substantially constant if the amount of SMA supported onthe carrier is 27% by weight or more. In other words, it is suggestedthat the catalytic activity substantially reaches saturation if SMA issupported in an amount sufficient to form a monolayer covering theentire surface of the silica carrier.

In Example 7, a 27 wt % SMA/SiO₂ catalyst was prepared as in Example 1by using each of silica powders 1, 2 and 3 prepared in PreparationExample 1, the silica powders differing from each other in specificsurface area, for evaluating the catalytic activity. The reaction wascarried out under the methane/oxygen ratio of the mixed gas of 9/1, thewater vapor flow rate of 3.5 L/h and the reaction temperature of 600° C.Table 5 and FIG. 6 show the results.

TABLE 5 Change in catalytic activity caused by change in specificsurface area of silica Specific 390 570 740 surface area (Silica (Silica(Silica of SiO₂ (m²/g) powder 1) powder 2) powder 3) Methane 6.4 11.312.1 conversion rate (%) Methanol/ 73.0 83.9 85.3 formaldehydeselectivity (%) CO 7.4 7.8 6.3 selectivity (%) CO₂ 19.6 9.0 8.3selectivity (%) Methanol/ 4.69 9.44 10.36 formaldehyde yield (%) *27 wt% SMA/SiO₂ *Methane/oxygen/water vapor flow rate (L/h): 1.8/0.2/3.5*Reaction temperature: 600° C.

As apparent from the experimental data, silica powder 1 having thesmallest specific surface area led to a low methane conversion rate andto a low selectivity of methanol/formaldehyde. As a result, the yield ofmethanol/formaldehyde for silica powder 1 was lower than those for theother silica powders. It is considered reasonable to understand that, inthe catalyst prepared by using silica powder 1, SMA molecules areagglomerated on the surface of the silica carrier. This clearly suggeststhat the SMA agglomeration is not desirable for the partial oxidation ofmethane.

The reactions were carried out at 600° C. in Examples 3, 5, 6 and 7.Then, the effect given by the reaction temperature to the catalyticactivity will now be described as Example 8.

Example 8 Dependence of Catalytic Activity on Reaction temperature

The reaction was carried out under the conditions similar to those forExample 7. In Example 8, used was 1.5 g of a 27 wt % SMA/SiO₂ catalystprepared by using silica powder 2, and the reaction temperature waschanged within a range of between 550° C. and 650° C. so as to evaluatethe effect given by the reaction temperature to the catalytic activity.Table 6 and FIG. 7 show the results.

TABLE 6 Change in catalytic activity caused by change in reactiontemperature Reaction temperature (° C.) 550 580 600 620 650 Methaneconversion 2.7 8.6 11.3 11.5 11.8 rate (%) Methanol/formal- 77.4 84.183.9 79.4 39.2 dehyde selectivity (%) CO selectivity (%) 16.8 10.5 7.87.5 21.3 CO₂ selectivity (%) 5.8 5.4 9.0 13.1 39.5 Methanol/formal- 2.097.23 9.44 9.13 4.62 dehyde yield (%) *27 wt % SMA/SiO₂, specific surfacearea of silica: 570 m²/g *Methane/oxygen/water vapor flow rate (L/h):1.8/0.2/3.5

The experimental data clearly support that the reaction of the presentinvention for synthesizing formaldehyde proceeds satisfactorily underthe reaction temperatures of 550° C. to 650° C. Particularly, where thereaction temperature fell within a range of between 580° C. and 620° C.,each of the methane conversion rate and the methanol/formaldehydeselectivity was found to be high, making it possible to obtainmethanol/formaldehyde at a high yield.

The durability of the catalyst will now be described as Example 9.

Example 9 Durability of Silica-Supported SMA Catalyst

Durability tests were conducted under different conditions by using 1.5g of a 27 wt % SMA/SiO₂ catalyst. Specifically, one of the tests wasconducted for 85 hours under the methane flow rate of 1.8 L/h, theoxygen flow rate of 0.2 L/h, the water vapor flow rate of 3.5 L/h andthe reaction temperature of 600° C. Table 7 and FIG. 8 show the results.

TABLE 7 Durability test using 27 wt % SMA/SiO₂ catalyst Reaction 1 17 3349 61 85 time (h) Methane 11.3 9.3 9.9 9.5 8.9 9.8 conversion rate (%)Methanol/ 83.5 83.2 84.6 82.1 87.6 85.2 formaldehyde selectivity (%)Methanol/ 9.44 7.52 8.36 7.77 7.17 8.39 formaldehyde yield (%) *27 wt %SMA/SiO₂, specific surface area of silica: 570 m²/g*Methane/oxygen/water vapor flow rate (L/h): 1.8/0.2/3.5 *Reactiontemperature: 600° C.

The other test was conducted for 330 hours under the methane flow rateof 1.2 L/h, the oxygen flow rate of 0.8 L/h, the water vapor flow rateof 3.5 L/h and the reaction temperature of 600° C. Table 8 and FIG. 9show the results.

TABLE 8 Durability test using 27 wt % SMA/SiO₂ catalyst Reaction 1 43.2100.4 193.3 250.0 330.0 time (h) Methane 25.2 20.0 20.1 20.5 19.6 24.5conver- sion rate (%) Methanol/ 80.5 93.1 86.5 91.1 90.8 85.1 formal-dehyde selec- tivity (%) Methanol/ 20.28 18.58 17.41 18.67 17.77 20.87formal- dehyde yield (%) Material 109.5 91.0 110.3 98.9 103.6 104.3balance (%) *27 wt % SMA/SiO₂, specific surface area of silica: 570 m²/g*Methane/oxygen/water vapor flow rate (L/h): 1.2/0.8/3.5 *Reactiontemperature: 600° C.

The experimental data clearly support that the catalyst of the presentinvention permits stably maintaining the methanol/formaldehyde yieldover 300 hours. It has been found that, although SMA is soluble inwater, SMA does not elute out of the catalyst layer even under the watervapor atmosphere so as to be held within the pores of the silicacarrier, leading to an excellent durability of the catalyst.

The material balance of carbon, which has been determined by the formulagiven below, is also shown in Table 8:

Material Balance of Carbon=A/B×100

where A represents the sum of the unreacted methane, methanol,formaldehyde, CO and CO₂, and B denotes the amount of methane suppliedto the reaction system.

The material balance fell within a range of between 90 and 110,supporting that the analysis was performed appropriately.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. A method of directly producing formaldehyde from a mixed gas of methane and oxygen in the presence of a catalyst, wherein said catalyst comprises a silica carrier and 12-molybdosilicic acid supported on the silica carrier, and water vapor is supplied to the mixed gas of methane and oxygen such that the water vapor occupies 40 to 80% by volume of the total volume of said mixed gas and water vapor.
 2. The method according to claim 1, wherein said 12-molybdosilicic acid is supported on the silica carrier in an amount of at least 10% by weight based on the silica amount.
 3. The method according to claim 1, wherein said silica carrier has a specific surface area of at least 500 m²/g.
 4. The method according to claim 2, wherein said silica carrier has a specific surface area of at least 500 m²/g.
 5. The method according to claim 1, wherein the volume ratio of methane/oxygen of said mixed gas falls within a range of between 9/1 and 4/6.
 6. The method according to claim 2, wherein the volume ratio of methane/oxygen of said mixed gas falls within a range of between 9/1 and 4/6.
 7. The method according to claim 3, wherein the volume ratio of methane/oxygen of said mixed gas falls within a range of between 9/1 and 4/6.
 8. The method according to claim 4, wherein the volume ratio of methane/oxygen of said mixed gas falls within a range of between 9/1 and 4/6.
 9. The method according to claim 1, wherein the reaction is carried out in the presence of said catalyst at 550 to 650° C.
 10. The method according to claim 2, wherein the reaction is carried out in the presence of said catalyst at 550 to 650° C.
 11. The method according to claim 3, wherein the reaction is carried out in the presence of said catalyst at 550 to 650° C.
 12. The method according to claim 4, wherein the reaction is carried out in the presence of said catalyst at 550 to 650° C.
 13. The method according to claim 5, wherein the reaction is carried out in the presence of said catalyst at 550 to 650° C.
 14. The method according to claim 6, wherein the reaction is carried out in the presence of said catalyst at 550 to 650° C.
 15. The method according to claim 7, wherein the reaction is carried out in the presence of said catalyst at 550 to 650° C.
 16. The method according to claim 8, wherein the reaction is carried out in the presence of said catalyst at 550 to 650° C. 